This invention relates to a method and apparatus for detecting and reducing the effects of ambient electromagnetic noise (including photic noise) on electronic instruments, particularly on electronic physiological monitoring instruments such as pulse oximeters.
The number of different kinds of electronic instruments used in hospitals, and the number of all electronic instruments of all kinds in use at any given time in each hospital, are on the rise. Besides performing its intended function, each instrument emits electromagnetic radiation at frequencies and intensities governed by the configuration of its electronic circuitry and the manner in which the instrument is used. For some instruments, such as radio telemetry monitors, the emission of electromagnetic radiation is the instruments' primary function.
In addition, superimposed on the electromagnetic radiation emitted by the instruments is the electromagnetic radiation emitted by the room lights and the A.C. power supply. In each room of the hospital, these electronic emissions combine to provide a complex background noise level whose instantaneous frequency and intensity characteristics depend on room lights, room power, and the nature of the instruments in use at any particular time. The effect of this background noise on the operation of an electronic instrument depends on the nature of the instrument. The use of a pulse oximeter in a noisy environment is a good example.
The principles of pulse oximetry and the operation of commercially available pulse oximeters are well known in the art. For example, the sensor of the pulse oximeter system described in U.S. Pat. No. 4,653,498 and U.S. Pat. No. 4,869,254 (both of which are incorporated herein by reference for all purposes) emits light alternately at a red and at an infrared wavelength into the patient's tissue, and a single photodetector senses the light transmitted through the tissue at each wavelength. The time-varying photodetector output represents the transmitted red and infrared signals separated by "dark" periods during which no light is emitted by the sensor. A demultiplexer synchronized with the sensor's red and infrared light sources separates the red and infrared portions of the photodetector output for further processing by the oximeter.
The physiological parameter measured by pulse oximeters is arterial blood oxygen saturation. The light-absorptive properties of blood at red and infrared wavelengths vary with the relative concentrations of oxyhemoglobin and deoxyhemoglobin in the blood. The portions of the photodetector output used in the oxygen saturation calculation, therefore, are the changes in red and infrared light transmission caused by the pulsatile changes in arterial blood volume at the sensor site. These pulse to pulse changes in transmitted light level are small in comparison to the overall intensity of the transmitted light, on the order of 1--3%, and are very susceptible to the influence of background noise.
The output of electric lights varies at a frequency related to the frequency of the A.C. power supply and its harmonics. If any frequency, fundamental or harmonic, of the ambient light variations match or are close to any frequency, fundamental or harmonic, of the oximeter's multiplexed light source, and if ambient light somehow reaches the photodetector, the oximeter may not be able to distinguish between the photodetector output related to red and infrared light sources (i.e., the signal) and the photodetector output related to ambient light (i.e., the noise). The red and infrared light are therefore typically multiplexed (and the photodetector synchronously demultiplexed) at frequencies other than room light frequencies. See, e.g., U.S. Pat. No. 4,653,498.
There are, however, many other sources of electromagnetic radiation in the pulse oximeter operating environment, including ECG monitors, impedance apnea monitors, isolation power supplies in other monitoring instruments, and electrocautery tools, each with its own characteristic operating frequencies. It would be difficult, if not impossible, to select an oximeter synchronous demultiplexer frequency that would not be affected by at least one of the potential noise sources in the oximeter's operating environment.
One prior art approach to this problem is to add a low-pass filter at the photodetector output to remove portions of the photodetector output signal above a certain frequency, say 100 to 300 kHz. See, e.g., U.S. Pat. No. Re. 33,643. This filter would not remove the effects of noise at the oximeter's synchronous demultiplexer frequency, however.
What is needed, therefore, is a way to reduce the effects of ambient electromagnetic noise in electronic monitoring instruments, especially when the noise source frequency (or a harmonic of the noise source frequency) is approximately the same as the fundamental frequency or harmonics at which the instrument is operating.